Recently, a multiple input multiple output (MIMO) system receives much attention to maximize the performance and communication capacity of a wireless communication system. Emerging from the use of a single transmission antenna and a single reception antenna as it has been to date, the MIMO technique employs multiple transmission antennas and multiple reception antennas to enhance data transmission/reception efficiency. The MIMO system, also called a multiple antenna system, is an application of a technique that collects data fragments received via several antennas for completion, rather than relying on a single antenna path, to receive a single overall message. As a result, a data transfer rate at a particular range can be improved or system coverage can be increased over a particular data transfer rate.
The MIMO technique includes transmit diversity, spatial multiplexing, beamforming, and the like. The transmit diversity is a technique that transmits the same data from multiple transmission antennas to thus enhance a transmission reliability. Spatial multiplexing is a technique that simultaneously transmits different data from multiple transmission antennas to thus transmit high speed data without increasing a bandwidth of a system. Beamforming is used to increase a signal to interference plus noise ratio (SINR) of a signal by applying a weight value according to a channel state at multiple antennas. In this case, the weight value may be represented by a weight vector or a weight matrix, and it is called a precoding vector or a precoding matrix.
spatial multiplexing includes spatial multiplexing for a single user and spatial multiplexing for multiple users. The spatial multiplexing for a single user is called a single user MIMO (SU-MIMO), and the spatial multiplexing for multiple users is called spatial division multiple access (SDMA) or multi-user MIMO (MU-MIMO). The capacity of a MIMO channel increases in proportion to the number of antennas. The MIMO channel may be disintegrated into independent channels. If the number of transmission antennas is Nt and the number of reception antennas is Nr, the number of independent channels Ni is Ni≦min{Nt, Nr}. Each independent channel may be a spatial layer. A rank is the number of non-zero eigenvalue of the MIMO channel, which may be defined as the number of spatial streams that can be multiplexed.
A synchronization signal is transmitted by a base station (BS) to align physical synchronization of time and frequency. The synchronization signal may indicate segment information and the ID (Identifier) of the BS by using a particular sequence. A midamble is a signal for a channel estimation transmitted by the BS in order to obtain a channel state of each antenna in the MIMO system using a plurality of antennas. A user equipment (UE) may receive the synchronization signal, be physically synchronized with the BS, obtain BS information, and receive a midamble to estimate a channel state of each antenna of the BS.
The synchronization signal may be transmitted via a plurality of synchronization channels (SCH). For example, the synchronization channel may be divided into a channel for aligning physical synchronization and a channel for indicating segment information and the ID of the BS. In the MIMO system, a method of using the synchronization signal as well as the midamble may be considered in order to obtain the channel state of each antenna, but has not been clearly proposed yet.
Thus, a method for estimating a channel state of each antenna by using a synchronization signal and a midamble in the MIMO system is required.